In recent years, application products typified by electronic equipment, automobile parts, etc. having higher performance generate heat in an increased heat amount from electronic parts installed therein. In addition thereto, miniaturization and thinning of such application products have been demanded, to thereby increase the heat generation density from such electronic parts. In order to maintain the functions of such application products and ensure the reliability thereof, such parts must be kept within appropriate temperatures. In particular, optical semiconductor elements such as light emitting diodes (LEDs) and laser diodes (LDs) to be used in electronic equipment, automobile parts, etc. may have properties in which the performance and life thereof may deteriorate due to heat generated by themselves. Further, wavelength converting materials such as phosphor to be used to change emission color in combination with a light emitting element may have properties in which the performance may deteriorate due to heat generated by the light emitting element. Therefore, application products utilizing such LEDs, LDs, phosphor, etc. should be appropriately controlled to be kept at suitable heat proof temperature or lower.
In general, a heating body is provided with a heat dissipation device such as a heat sink with a mechanism in which heat generated by the heating body can be dissipated to surrounding air. The heat dissipation performance of such a heat sink can be determined on the basis of an envelope volume thereof. The larger the envelope volume is, the higher the intrinsic heat dissipation performance becomes. On the other hand, simply enlarged envelope volume may impair the miniaturization and thinning demands of such products, and thus, it would be difficult to simply increase the envelope volume. Thus, it is an important object to effectively dissipate heat of a heating body into air with a limited space (volume). Furthermore, it is also a recently important object to decrease a weight of a heat dissipation device without reducing a heat dissipation performance of the heat dissipation device with unchanged volume because of weight-saving demands.
Under these circumstances, there has been proposed a technique disclosed in Japanese Patent Application Laid-Open No. 2009-099878, in which the weight of a heat sink is reduced by using a laminate of a graphite sheet and a metal plate as a fin.
Graphite used in the heat sink disclosed in Japanese Patent Application Laid-Open No. 2009-099878 has a high heat conductivity, and thus has been expected to show a higher heat dissipation performance. The used metal thin plate can impart a rigidity to the graphite used, thus enabling the use of a graphite sheet. This can further contribute to the thinning and weight saving effects.
Even in this case where the heat sink includes a fin of graphite, however, a metal base having a high heat conductivity and a high rigidity is required to constitute the heat sink. Thus, it is impossible to perform an integral mold of the metal plate of the fin and the metal base as in the case of a general heat sink. Such a graphite-made fin and a metal base must be structurally and thermally connected to each other by any suitable means. In this context, the technique disclosed in Japanese Patent Application Laid-Open No. 2009-099878 uses a solder to bond the metal thin plate of the fin and the metal base.
Furthermore, such a base is desired to have a film with a high heat dissipation performance on its surface to improve the heat dissipation characteristics of the base itself. For example, when the base is made of aluminum, the base is subjected to an anodization process or the like to form an alumite film on its surface. Another method may include performing application of a film having a high heat dissipation property.
In order to achieve a lightweight heat sink with a higher heat dissipation performance, the inventor of this invention has confirmed a heat dissipation performance of a heat sink including a base on the surface of which a high heat dissipation film was provided and a fin formed from a laminate of graphite and a metal thin film by conducting experiments and computer simulations based on the computational fluid dynamics. As a result, the inventor has found that it was difficult to obtain a high heat dissipation performance of such a heat sink. The cause thereof has been found that the thermal resistance of the connection part between the base and the fin was too large to effectively conduct heat from the base to the graphite sheet of the fin.